This invention relates to a method of reducing the SO.sub.x emission in the combustion of coal which contains sulfur compounds and more particularly, to a three zone furnace for burning high sulfur coal.
The use of coal for the generation of power and the like is increasingly important as supplies of hydrocarbons become scarce. The use of coal has been limited by the high sulfur content of much coal. When burned, high sulfur coal produces SO.sub.3 and SO.sub.2 which have an adverse environmental impact.
Attempts to minimize this impact have included extensive cleaning of the sulfur coals prior to burning. For example, in U.S. Pat. No. 4,052,170-Yan, paramagnetic impurities including sulfur compounds are magnetically separated from the coal before burning. U.S. Pat. Nos. 4,077,866 Owen, Venuto and Yan and 4,118,201-Yan show other techniques for producing low sulfur fuel from coal.
Another approach is to remove SO.sub.3 and SO.sub.2 from the flue gas emitted from the furnace. Some of the techniques for accomplishing this are described in "SULFUR DIOXIDE EMISSION CONTROL BY HYDROGEN SULFIDE REACTION IN AQUEOUS SOLUTION," Bureau of Mines Report of Investigations/1973, R. I. 7774, United States Department of the Interior.
Desulfurization has also been practiced during the conversion of coal to its volatile components, an example of which is shown in U.S. Pat. Nos. 3,736,233 Sass et al., in 4,253,409 Warmser and 3,727,562 Bauer.
The equipment which is required to separate the sulfur from the coal prior to burning and to collect SO.sub.x emissions from the flue gas is very expensive. This has limited the use of these approaches to controlling SO.sub.x emission.
Another approach which does not require the use of expensive equipment is to add scavengers to the coal. See U.S. Pat. Nos. 4,245,573-Dixit et al, 4,256,703-Dixit et al and 4,322,218-Nozaki. It is known that a significant amount of sulfur will be retained in the ash if the coal contains basic minerals, such as calcium carbonate, dolomite, or soda ash. These basic minerals act as scavengers which trap the SO.sub.x, preventing its emission, and facilitating removal with the ash.
It is an object of the present invention to improve the trapping efficiency of sulfur scavengers, both indigenous in the coal or added to it.
In accordance with the present invention a, multiple stage combustion furnace is operated in a manner which facilitates the removal of ash containing sulfur from the combustion process.